Semiconductor materials are typically formed by epitaxial growth on a substrate. When a crystalline substrate is used the crystal structure of the epitaxially grown semiconductor aligns with the crystal structure of the substrate. However, when the epitaxially grown semiconductor comprises atoms which differ in size from the atoms of the substrate, strain is introduced in the epitaxially grown material.
One example is silicon germanium grown on a silicon substrate. The silicon germanium crystal lattice aligns with the silicon crystal lattice. However, the germanium atoms are larger than the silicon atoms so the silicon germanium material is compressively strained.
Strained semiconductors are valued due to better electron mobility and hence better performance. However, defects can arrive in strained materials which have a negative effect on semiconductor device performance.